Principles for the use of phototransistor optocouplers

Würth Elektronik publishes Application Note on optocouplers

by gabi

Würth Elektronik has expanded its collection of practical information to support electronics developers with the new AppNote ANO007 “Understanding Phototransistor Optocouplers” ( Optocouplers, also known as photocouplers or optoisolators, are components that send an electrical signal by means of optical coupling between two galvanically separated circuits. An essential aspect for the development of robust applications involving optocouplers is not only to understand and integrate their main parameters, but also to take the potential effects of their parasitic elements into consideration. A profound understanding of these aspects and the precise consideration of tolerances, deviations, and interdependences are indispensable. The new Application Note starts by explaining the principles and goes on to present comprehensive experimental results gained with the WL-OCPT optocouplers made by Würth Elektronik.

Optocouplers offer high isolation voltage (5 kV) in connection with a very compact form factor. In contrast to transformers and capacitors, which are only capable of transmitting AC signals, the optocouplers send both AC and DC signals. This makes them particularly attractive for applications such as isolated communication interfaces, high-voltage motor drives, AC grid monitoring, control circuits for power-supply stations, industrial I/O modules, or sensor systems. The AppNote provides instructions on the characterization of parameters and on important aspects that must be considered from the design aspect.


  • Design and function of optocouplers
  • DC and AC CTR: Calculating the CTR scope
  • Parasitic Capacitance and cut-off frequency
  • Switching behaviour and DC biasing
  • Parameter measurement and characterization
  • Experimental results for the WL-OCPT 817-816 series

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